A device or system may be implanted into a patient to control movement between two bones or tissue portions. For example, bone implants and systems are used for, among other purposes, control and stabilization of the posterior lumbar spine. In the case of spinal degeneration, for example, of a disc or a vertebra, the spine may be unstable, and undesired motion may be possible. In such a case, it is known to use a bone implant or system to stabilize the spine while still allowing some controlled motion.
Typical spinal systems include pedicle screws that attach to adjacent vertebrae; rigid or semi-rigid rods or plates that extend between the screws of adjacent vertebrae; and connectors for connecting the rods or plates with the screws. Some systems are designed not to allow for any relative movement between vertebrae. Other systems allow for some relative movement between vertebrae, such as via pivotal connectors and/or flexible rods or plates, in an attempt to allow some controlled movement of the spine while still stabilizing the spine.